In the development and production of devices for high-frequency communications systems, such as digital mobile telephone or WLAN (wireless local area network), it is desirable to check the performance of the relevant devices or component groups with regard to data transmission or message transmission. Measuring instruments connected directly by means of a cable to the antenna port of the device under test (DUT) are generally used for this purpose. If the device under test has only a permanently-installed internal antenna, an antenna coupler, which establishes the connection with the antenna of the device under test by electromagnetic coupling, is inserted between the device under test and the end of the cable.
The high-frequency signals to be transmitted or respectively received are passed along the cable. In order to test a mobile-telephone device, for example, bit sequences are generated by the measuring device and transmitted after appropriate modulation to the mobile-telephone device under test, wherein the corresponding transmitter unit of the measuring device adjusts various parameters, such as the level or the frequency of the transmitted signal, in order to observe given test conditions. For example, one measuring device of this kind emulates a base station of a real mobile telephone network, so that the measuring device comprises high-frequency transmitter and/or receiver devices as integral components in addition to the components actually required to implement a test run.
The measuring device in a test station used for testing the devices in a high-frequency communications system is generally installed in a rack, which also generally contains other measuring devices for implementing other measuring tasks. The high-frequency signals received from the device, and the high-frequency signals generated and transmitted by the device are transmitted respectively from or to the device under test via a cable connection.
Transmitting high-frequency signals via cable is associated in principle with substantial disadvantages. The attenuation of the cable is dependent upon the length of the cable itself, the signal frequency and the type of cable and influences the accuracy, with which signals are supplied to the device under test, and also the accuracy, with which transmissions from the device under test can be measured. Complex corrective calculation methods can reduce these effects in the determination of the true measured values, but can never completely eliminate them. A further difficulty is that the high-frequency properties of a cable can change over time, for example as a result of mechanical stress. On the one hand, the attenuation of the cable means that the measuring device must generate a higher level than is required directly in the device under test, which, especially in the case of high-frequencies, makes the measuring devices more expensive. Weak signals transmitted from the device under test may fall below the detection threshold of the measuring device as a result of the cable attenuation, so that, under some circumstances, more expensive, sensitive measuring devices are required.
With an increasing length of cable, not only are these effects intensified, but the risk is also increased that interference signals (for example, from the base stations disposed in the proximity of the building, in which the test station is located) can penetrate through the final shielding of the cable and thereby falsify the measurement.
Furthermore, cables generally have a negative effect on the standing wave ratio (VSWR, voltage standing wave ratio) of measuring devices and therefore lead to additional measurement and stimulus uncertainties.
In summary, for the above reasons, the length of the cable between the device under test and the measuring device should be kept as short as possible. However, this is in contradiction with conventional practice at test stations, which demands a spatial separation of the device under test from the measuring device for reasons of space, for example, for conveyor belts, handling systems, pneumatically-controlled test adapters, and with regard to the space requirements of the measuring devices themselves.
In view of the rapidly-changing technology, measuring devices must become increasingly universal and future-orientated. It must be possible to expand the devices for functions, which are currently not yet required or which, under some circumstances, are not even known at the time of purchasing the device. Examples of such expansions could be the covering of further frequency and level ranges, new mobile-telephone standards, the number of independently usable transmission and reception models, for example, in order to test more than one device under test at the same time.
For specialized applications, it is sometimes also meaningful to derive devices with a reduced performance scope and at lower cost from existing measuring-device designs. Possible solutions in this context may be found in the modularity of the measuring device, as disclosed, for example, in published German patent application DE 198 57 834 A1. However, plug-in designs are subject to limitations, which are determined by the availability of space, the heat removable from the device, which determines the maximum-permissible power consumption of the modules, and is determined by the performance of the power pack, which is generally installed in a fixed manner.